Known hydraulic actuation systems may include two servo actuator arrangements, each being arranged to drive opposite ends or different regions of a common element. Each servo actuator arrangement includes a linear actuator having first and second opposing end chambers. The pressure of fluid within the first and second chambers, and hence the position of the actuator, is controlled by means of an electrohydraulic servo valve. The servo valve is supplied with an electrical current which energises a winding of the servo valve to control the position of a spool valve and, hence, the flow of fluid to the first and second chambers of the actuator.
In some applications, it has been recognised that the use of electric actuator systems, as opposed to hydraulic systems, offers the potential for increased reliability and efficiency and reduced weight and manufacturing cost. One type of actuation system that uses direct electrical power for actuation is commonly referred to as an electrohydrostatic or electro hydraulic actuator (EHA). Another type is an electromechanical actuator (EMA). Both EHA and EMA systems employ a servomotor arrangement to control motor operation so as to position a linear actuator. Both systems rely on changes of motor speed and direction to convert rotary energy produced by the electric motor to linear actuator energy. In an EHA system, the energy transformation is through a hydraulic medium charged by a fixed displacement pump, whereas in an EMA system the transformation is by mechanical means.
Another type of electric actuation system is an Integrated Actuation Package (IAP) such as that described in U.S. Ser. No. 60/092,942, in which the control of the displacement of a hydraulic pump (i.e. the control of hydraulic flow rate and flow direction) is used to position a linear actuator. IAP systems typically include a fixed speed, unidirectional electric motor powered by an electrical source, a hydraulic servo pump and a linear hydraulic actuator. A control system is arranged to control the position of the actuator by changing the displacement of the servo pump to vary the hydraulic pump output flow rate and flow direction in response to actuator command signals and position error signals.
As an intermediate step towards using only electric actuation systems, for some applications it has been proposed to drive an object by means of a dual servo/electric actuator arrangement, the servo actuator being arranged to drive a first part of the object and the electric actuator being arranged to drive another part of the object remotely spaced from the first part. However, due to the different operating characteristics of the electric actuator and the servo actuator, there can be undesirable differences in the load applied by each actuator and the relative timing of the applied loads. This can result in excessive energy waste and undesirable stresses being applied to the driven object.